This invention relates to a method and an apparatus which allows, at any point of navigation of a tanker and regardless of variation in the velocity of the tanker, the determination of the oil content in deballasting water and tank cleaning water that has been discharged during the navigation of a distance or range extending from said point of navigation to another point with a given distance backward thereof. Both at national and international levels, the importance is being recognized of laying down regulations for preventing marine pollution. According to the regulations concluded by the Inter-Governmental Marine Consultative Organization (IMCO), which is the specialized agency of the United Nations, all discharge into the sea of deballasting water and tank cleaning water from tankers is prohibited except when all the following conditions are satisified:
(1) The tanker is not within a special area; PA1 (2) The tanker is more than 50 nautical miles from the base line of the territorial waters of the nearest nation; PA1 (3) The tanker is proceeding en route; PA1 (4) The instantaneous rate of discharge of oil content in deballasting water and tank cleaning water does not exceed 60 liters per nautical mile (the instantaneous rate of discharge of oil content is the total quantity of oil discharged per hour divided by the velocity of the tanker in terms of "knot" and this is hereunder referred to as an incremental quantity of oil discharged); PA1 (5) The total quantity of oil discharged does not exceed for existing tankers 1/15,000 of the total cargo carrying capacity of the vessel, and for new tankers 1/30,000 of the total cargo carrying capacity of the vessel; and PA1 (6) The tanker has, in operation, oil discharge monitoring and control systems.
This invention relates to a method and an apparatus for determination of the oil content in deballasting water or cleaning water discharged from a travelling tanker, thereby controlling the discharge of oil in such a manner that the condition (4) mentioned above may be satisfied. Conventionally, the quantity of oil content discharged is controlled by first multiplying an instantaneous oil concentration p by an instantaneous flow amount q and dividing the product by the velocity of an associated tanker to obtain an incremental quantity of oil discharged L + pg/V (the instantaneous rate of oil discharged as defined in the condition (4)) and then stopping the discharge of deballasting water and washing water from the tanker when said value exceeds a predetermined level (for example, 60 liters/nautical mile as mentioned earlier). But in view of the nature of oil (it is easily deposited on the sampling line), this method is not acceptable. To be more specific, once concentrated oil is deposited on the sampling line, instantaneous determination gives a value L = pg/V that is greater than the allowable limit, and then this requires the discharge of deballasting water and cleaning water to be stopped, but then q becomes zero to give L=O that is smaller than the allowable limit, and this enables the deballasting water and cleaning water to be discharged again. However, the oil concentration is still high enough to make L greater than the allowable limit and therefore the discharge is stopped again, whereupon an oscillation or chattering effect occurs to cause a failure in control of the discharging operation.
According to another method that has been devised to overcome this defect, an oil discharge rate L for a predetermined distance (for instance, a nautical mile) is determined by the following formula, with the velocity V of a tanker and the distance l being constant and an instantaneous concentration of oil p and an instantaneous flow rate q being actually measured: ##EQU1## (wherein V and l are constant in l/V = t, and therefore, t is constant). However, this method requires the factors to be reset upon every lapse of the time "t" required for travelling the predetermined distance (for instance, one nautical mile). For this reason, determination is also required at places (or times) other than the set places (or times) of "t.sub.1 " and "t.sub.2 " (see FIG. 9). In addition, it is technically difficult to maintain the navigating velocity of a vessel constant.
Such drawbacks inherent in the conventional methods for controlling the discharge of oil content are eliminated by the method and apparatus of the present invention. In the pages that follow, some embodiments of this invention will be described in detail based on the attached drawings.